Craniofacial Muscles

14 Tongue Muscle Response to Neuromuscular Diseases and Speci fi c Pathologies
259
tongue which is thought to produce greater forces than does the tongue base
(Pouderoux and Kahrilas 1995 ) . Our in vivo loading study revealed that the tongue
produces more load in mandibular lingual surfaces than the premaxillary and maxillary
palatal surfaces, and these loads decrease in the anterior mouth (symphysis and
premaxilla) after the mass reduction. Loads in the posterior mouth (mandibular corpus
and posterior maxillary palatal surface) are less affected (Liu et al . 2008b ) .
Therefore, the observed slow growth in the skeletal components may in part contribute
to the decrease of functional loads in the anterior mouth by a mass-reduced
tongue. Second, among affected components of the craniofacial skeleton, the mandible
is affected more than the nasomaxillary skeleton in all dimensions: length,
width, and height. This striking difference between upper and lower jaws was also
con fi rmed by the examination of bone mineral density in which the only signi fi cant
decrease was found in the mandibular symphysis bloc of the mass-reduced animals
(Liu et al . 2008a ) . Anatomically, the tongue is directly attached to the mandible
through its musculature. Functionally, there is an inherent linkage between the
tongue and mandible (Palmer et al . 1997 ) . Furthermore, the mechanism of cranial/
nasomaxillary postnatal growth is mostly attributed to sutures, different from that of
the mandible which mainly depends on appositional deposition through intramembranous
ossi fi cation at the borders and alveolar ridges, and on secondary cartilage
through endochondrous ossi fi cation at the condyle (Sarnat 1997 ; Kantomaa and
Ronning 1997 ) . Based on these fi ndings, it is not surprising that the postnatal mandibular
growth would be suppressed more than the other elements of the craniofacial
skeletons by the tongue mass reduction.
14.4 Conclusions
Thus, compared to body skeletal muscles, the tongue musculature presents striking
differences with the following features: (1) myo fi bers of extrinsic (with bony attachment)
and intrinsic (without bony attachment) tongue muscles are aligned in both
parallel (longitudinal) and perpendicular (transverse, vertical, circumferential, or
radial) directions, and interweave with each other. This forms an intricate array and
provides the basis for multidirectional contraction and regional-dependent deformation;
(2) the unique architecture of the tongue musculature grants this organ an
enormous biomechanical versatility to ful fi ll various functional demands, and the
tongue motor control most likely uses grouped motor unit- or segmented structure
unit-based, rather than entirely muscle-based strategies. Thus, the tongue kinematics
in regions may not apply to the widely held theory of a muscular hydrostat;
(3) the extrinsic and intrinsic tongue muscles are not only structurally interwoven
but also functionally interacting. The reversals of expansion–contraction of various
dimensions of the tongue are not synchronous but occur in a sequential manner as a
function of performing tasks; (4) changes in the tongue mass not only signi fi cantly
alter the pattern of tongue kinematics, but manifest biomechanical effects on surrounding
hard tissue, which in turn affects the growth of the craniofacial skeleton